Several systems have been provided for use by aircraft or sea-going vessels or the like in determining position with respect to a stationary object or reference point. In most systems, the user desiring to fix position with respect to one or more reference points radiates an electromagnetic signal and calculate range or bearing measurements with two different reference points and thereafter compute the user's position by triangulation or similar mathematical techniques. While such triangulation methods are technically sound, they are time consuming and may detract the user's attention from other required activities such as piloting the aircraft, etc. In addition, techniques which require that the user radiate an electromagnetic signal are costly and often complex. In addition, such systems are unsuitable for use in military environments in which the user desires to remain undetected.
A variety of imaging systems are available which utilize a cathode ray tube display often having overlays of maps, etc. in combination with a radar system. While the systems are in some respects effective, they often fail to provide the user with knowledge of the user's position within the cathode ray tube image.
The need for accurate systems for position fixing by aircraft and sea-going vessels has prompted practitioners in the art to develop a variety of systems. For example, U.S. Pat. No. 3,742,504 issued to Shores sets forth an AIRCRAFT COLLISION AVOIDANCE SYSTEM BY PASSING MEANS in which a passive collision avoidance system applicable to aircraft collision avoidance includes two receivers operating in combination with antenna having differing sensitivity to the polarization of the incoming signals from a ground broadcast facility. The differing sensitivity ensures that the direct end reflected signals will predominate in different receivers and therefore, when the outputs of the receivers are combined, the resultant fade rate will give an indication of movement of the intruding aircraft with respect to the equipped aircraft.
U.S. Pat. No. 4,293,857 issued to Baldwin sets forth a COLLISION AVOIDANCE WARNING SYSTEM utilizing a known location navigational ground station to enable a primary aircraft to determine its own position and employing a known location air surveillance radar including an interrogator to determine the location, velocity, course and altitude of a potentially conflicting other aircraft. The technique uses the time between the inception of an interrogating pulse and the reception of a transponder response from the target aircraft to compute the position and altitude of the aircraft.
U.S. Pat. No. Re. 32,368 reissued to Funatsu, et al. sets forth a COLLISION AVOIDANCE SYSTEM FOR AIRCRAFT in which one aircraft is equipped with an interrogation station having a secondary surveillance radar. A distance measurement is effected either by passive or active distance measurements or by both. If the distance falls within a certain limit, the output power and/or period of the interrogation signal of the secondary surveillance radar of the subject aircraft is altered.
U.S. Pat. No. 4,782,450 issued to Flax sets forth a METHOD AND APPARATUS FOR PASSIVE AIRBORNE COLLISION AVOIDANCE AND NAVIGATION for use as an onboard locating apparatus to monitor interrogation signals and ISLS signals transmitted by at least one secondary surveillance radar and coded reply signals transmitted by at least one fixed ground transponder to match the transponder reply signals with the ISLS signals. As a result, the system is able to associate the transponder signals with interrogations from the radar and repeatedly compute the aircraft's instantaneous position in space. The computations are based upon hyperbolic and linear functions. U.S. Pat. Nos. 4,107,674 and 4,196,434 both issued to Funatsu, et al. set forth related disclosures similar to that set forth in the above-described reissue U.S. Pat. No. Re. 32,368.
U.S. Pat. No. 4,835,537 issued to Manion sets forth TELEMETRY BURST COLLISION AVOIDANCE SYSTEM which provides warning and avoidance maneuvers for all fixed and moving obstructions which threaten the safe navigation of the host aircraft. The system is effective against threatening aircraft, runway maintenance vehicles and prominent geographic obstructions such as radio towers and mountain peaks. The system is capable of simultaneously broadcasting its host location and intended movement while receiving the same information from all nearby similarly equipped stations and aircraft.
U.S. Pat. No. 3,895,382 issued to Litchford sets forth a METHOD AND APPARATUS FOR MEASURING PASSIVELY RANGE AND BEARING in which apparatus for determining passively the slant range and bearing angle to another transponder equipped aircraft within a selectable proximity to a host aircraft equipped with the collision avoidance system is provided. The azimuthal lines of position from interrogating standard secondary surveillance radar ground stations to both aircraft and the times of arrival at the host aircraft of the transponder replies of other aircraft are determined. From such data the bearing angles to the actual and imaginary locations of the transponder equipped aircraft are calculated.
U.S. Pat. No. 4,380,050 issued to Tanner sets forth an AIRCRAFT LOCATION AND COLLISION AVOIDANCE SYSTEM in which the azimuth and range information of an aircraft with respect to a reference ground station is made available to other aircraft by transmission of a pulse at a time uniquely associated with the aircraft's location. A synthetic azimuth function and synthetic range function provide a periodic mapping of an area. The synthetic azimuth function is a slowed time expanded representation of a conventional azimuth function. Each azimuth increment is allocated a time slot in the synthetic azimuth function. The synthetic azimuth function is synchronized by counting a number of synchronizing pulses from a conventional azimuth function and a synthetic azimuth reference pulse is periodically transmitted from a reference ground station to synchronize all aircraft using the synthetic azimuth function.
While the foregoing described prior art devices have provided some measure of success in position fixing and collision avoidance, they are often complex and require substantial equipment both on the ground and in the various interacting aircraft. Thus, there remains a continuing need in the art for lower cost, simpler and more convenient to use position fixing systems suitable for aircraft or vessels at sea or the like.